Add sys/ucontext.h

* Move ucontext_t and mcontext_t there as that's where POSIX says they
should be.

headers/posix/sys/ucontext.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright 2014, Haiku, Inc. All Rights Reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _UCONTEXT_H_
+#define _UCONTEXT_H_
+
+
+typedef struct vregs mcontext_t;
+
+typedef struct __ucontext_t {
+	struct __ucontext_t*	uc_link;
+	sigset_t				uc_sigmask;
+	stack_t					uc_stack;
+	mcontext_t				uc_mcontext;
+} ucontext_t;
+
+
+#endif